Phosphorus stabilizer compounds such as alkyl phosphates and phosphoric acid are commonly added to polyester reaction mixtures as essential components in achieving low color and high stability in polyester polymers. Antimony compounds such as antimony trioxide, antimony triacetate and antimony ethylene glycoxide are commonly used polycondensation catalysts for high clarity polyester manufacturing. Antimony catalysts are also considered essential in the commercial production of polyesters, especially poly(ethylene terephthalate) or its copolymers. However, antimony phosphate particles are formed in the polyester when both antimony and phosphate ions are present during polymerization. The presence of antimony phosphate particles large enough to scatter light (particle diameter greater than about 200 nm) causes haziness in the polyester. An additional detrimental effect of large particles is that larger antimony phosphate particles cause a decrease in the catalytic activity of antimony, thus slowing down the rate of polymer production.
U.S. Pat. No. 4,408,004 and U.S. Pat. No. 4,499,226 teach that polyester polymer haze is somewhat reduced when the antimony catalyst compound is added during the condensation stage of the polymerization process and the phosphorus stabilizer compound is added at an earlier point, at least during early condensation, but preferably during the beginning of the esterification stage. However, the polyester made by such methods is still undesirably hazy.
In addition to the undesirable amount of haze provided by known methods, it is commercially undesirable to be confined to such an inflexible process. It would be better if the point of addition of materials such as stabilizers and catalysts could be determined mainly by process economics so that less energy capital is required to carry out the process. For example, it is particularly inefficient to add a solution of catalyst materials during the polycondensation stage because more solvent needs to be removed in the polycondensation stage than is normally designed for the process.
In light of the above, it would be desirable to provide a process for producing an antimony phosphate-containing polyester having a higher clarity than previously possible by known processes. It would be further desirable if such process were flexible enough so that the points of addition of the antimony catalyst compound and phosphorus stabilizer compound were very flexible. It would be even furter desirable for such process to be more economical and have a higher production rate than previously known antimony and phosphorus based polyester processes.